Comparing the Electronic Structure of Iron, Cobalt, and Nickel Compounds That Feature a Phosphine-Substituted Bis(imino)pyridine Chelate

It was recently discovered that (^Ph2PPrPDI)Mn (PDI = pyridine diimine) exists as a superposition of low-spin Mn(II) that is supported by a PDI dianion and intermediate-spin Mn(II) that is antiferromagnetically coupled to a triplet PDI dianion, a finding that encouraged the synthesis and electronic structure evaluation of late first row metal variants that feature the same chelate. The addition of ^Ph2PPrPDI to FeBr₂ resulted in bromide dissociation and the formation of [(^Ph2PPrPDI)FeBr][Br]. Reduction of this precursor using excess sodium amalgam afforded (^Ph2PPrPDI)Fe, which possesses an Fe(II) center that is supported by a dianionic PDI ligand. Similarly, reduction of a premixed solution of ^Ph2PPrPDI and CoCl₂ yielded the cobalt analog, (^Ph2PPrPDI)Co. EPR spectroscopy and density functional theory calculations revealed that this compound features a high-spin Co(I) center that is antiferromagnetically coupled to a PDI radical anion. The addition of ^Ph2PPrPDI to Ni(COD)₂ resulted in ligand displacement and the formation of (^Ph2PPrPDI)Ni, which was found to possess a pendent phosphine group. Single-crystal X-ray diffraction, CASSCF calculations, and EPR spectroscopy indicate that (^Ph2PPrPDI)Ni is best described as having a Ni(II)-PDI^2- configuration. The electronic differences between these compounds are highlighted, and a computational analysis of ^Ph2PPrPDI denticity has revealed the thermodynamic penalties associated with phosphine dissociation from 5-coordinate (^Ph2PPrPDI)Mn, (^Ph2PPrPDI)Fe, and (^Ph2PPrPDI)Co.

Multi PCET in symmetrically substituted benzimidazoles

Proton-coupled electron transfer (PCET) reactions depend on the hydrogen-bond connectivity between sites of proton donors and acceptors. The 2-(2′-hydroxyphenyl) benzimidazole (BIP) based systems, which mimic the natural Tyr_Z-His190 pair of Photosystem II, have been useful for understanding the associated PCET process triggered by one-electron oxidation of the phenol. Substitution of the benzimidazole by an appropriate terminal proton acceptor (TPA) group allows for two-proton translocations. However, the prototropic properties of substituted benzimidazole rings and rotation around the bond linking the phenol and the benzimidazole can lead to isomers that interrupt the intramolecular hydrogen-bonded network and thereby prevent a second proton translocation. Herein, a strategic symmetrization of a benzimidazole based system with two identical TPAs yields an uninterrupted network of intramolecular hydrogen bonds regardless of the isomeric form. NMR data confirms the presence of a single isomeric form in the disubstituted system but not in the monosubstituted system in certain solvents. Infrared spectroelectrochemistry demonstrates a two-proton transfer process associated with the oxidation of the phenol occurring at a lower redox potential in the disubstituted system relative to its monosubstituted analogue. Computational studies support these findings and show that the disubstituted system stabilizes the oxidized two-proton transfer product through the formation of a bifurcated hydrogen bond. Considering the prototropic properties of the benzimidazole heterocycle in the context of multiple PCET will improve the next generation of novel, bioinspired constructs built by concatenated units of benzimidazoles, thus allowing proton translocations at nanoscale length.

Proton-coupled electron transfer (PCET) reactions depend on the hydrogen-bond connectivity between sites of proton donors and acceptors.

Reaction of a Molybdenum Bis(dinitrogen) Complex with Carbon Dioxide: A Combined Experimental and Computational Investigation

Refluxing Mo(CO)₆ in the presence of the phosphine-functionalized α-diimine ligand ^Ph2PPrDI allowed for substitution and formation of the dicarbonyl complex, (^Ph2PPrDI)Mo(CO)₂. Oxidation with I₂ followed by heating resulted in further CO dissociation and isolation of the corresponding diiodide complex, (^Ph2PPrDI)MoI₂. Reduction of this complex under a N₂ atmosphere afforded the corresponding bis(dinitrogen) complex, (^Ph2PPrDI)Mo(N₂)₂. The solid-state structures of all three compounds were found to feature a tetradentate chelate and cis-monodentate ligands. Notably, the addition of CO₂ to (^Ph2PPrDI)Mo(N₂)₂ is proposed to result in head-to-tail CO₂ coupling to generate the corresponding metallacycle and ultimately a mixture of (^Ph2PPrDI)Mo(CO)₂ and the bis(oxo) dimer, [(κ³-^Ph2PPrDI)Mo(O)(μ-O)]₂. Computational studies have been performed to gain insight into the reaction and evaluate the importance of cis-coordination sites for selective head-to-tail CO₂ reductive coupling, CO deinsertion, disproportionation, and stepwise CO₂ deinsertion.

The electronic structure of a β-diketiminate manganese hydride dimer

The electronic structure of a dimeric manganese hydride catalyst supported by β-diketiminate ligands, [(2,6-iPr2PhBDI)Mn(μ-H)]2, was investigated with density functional theory. A triple bond between the manganese centres was anticipated from simple electron-counting rules; however, calculations revealed Mn-Mn Mayer bond orders of 0.21 and 0.27 for the ferromagnetically-coupled and antiferromagnetically-coupled extremes, respectively. In accordance with experimentally determined Heisenberg exchange coupling constants of -15 ± 0.1 cm-1 (SQUID) and -10.2 ± 0.7 cm-1 (EPR), the calculated J0 value of -10.9 cm-1 confirmed that the ground state involves antiferromagnetic coupling between high spin Mn(ii)-d5 centres. The effect of steric bulk on the bond order was examined via a model study with the least sterically-demanding version of the β-diketiminate ligand and was found to be negligible. Mixing between metal- and β-diketiminate-based orbitals was found to be responsible for the absence of a metal-metal multiple bond. The bridging hydrides give rise to a relatively close positioning of the metal centres, while bridging atoms possessing 2p orbitals result in longer Mn-Mn distances and more stable dimers. The synthesis and characterization of the bridging hydroxide variant, [(2,6-iPr2PhBDI)Mn(μ-OH)]2, provides experimental support for these assessments.

Proton-coupled electron transfer across benzimidazole bridges in bioinspired proton wires

Designing molecular platforms for controlling proton and electron movement in artificial photosynthetic systems is crucial to efficient catalysis and solar energy conversion. The transfer of both protons and electrons during a reaction is known as proton-coupled electron transfer (PCET) and is used by nature in myriad ways to provide low overpotential pathways for redox reactions and redox leveling, as well as to generate bioenergetic proton currents. Herein, we describe theoretical and electrochemical studies of a series of bioinspired benzimidazole-phenol (BIP) derivatives and a series of dibenzimidazole-phenol (BI₂P) analogs with each series bearing the same set of terminal proton-accepting (TPA) groups. The set of TPAs spans more than 6 pK_a units. These compounds have been designed to explore the role of the bridging benzimidazole(s) in a one-electron oxidation process coupled to intramolecular proton translocation across either two (the BIP series) or three (the BI₂P series) acid/base sites. These molecular constructs feature an electrochemically active phenol connected to the TPA group through a benzimidazole-based bridge, which together with the phenol and TPA group form a covalent framework supporting a Grotthuss-type hydrogen-bonded network. Infrared spectroelectrochemistry demonstrates that upon oxidation of the phenol, protons translocate across this well-defined hydrogen-bonded network to a TPA group. The experimental data show the benzimidazole bridges are non-innocent participants in the PCET process in that the addition of each benzimidazole unit lowers the redox potential of the phenoxyl radical/phenol couple by 60 mV, regardless of the nature of the TPA group. Using a series of hypothetical thermodynamic steps, density functional theory calculations correctly predicted the dependence of the redox potential of the phenoxyl radical/phenol couple on the nature of the final protonated species and provided insight into the thermodynamic role of dibenzimidazole units in the PCET process. This information is crucial for developing molecular “dry proton wires” with these moieties, which can transfer protons via a Grotthuss-type mechanism over long distances without the intervention of water molecules.

Experimental and theoretical methods characterize the thermodynamics of electrochemically driven proton-coupled electron transfer processes in bioinspired constructs involving multiple proton translocations over Grotthus-type proton wires.

Solution and Solid-State Characterization of PbSe Precursors

The addition of lead to diphenyl diselenide in ethylenediamine (en) or pyridine (py) allowed for the observation of the solvento complexes, (en)Pb(SePh)₂ or (py)₂Pb(SePh)₂, respectively. Performing this reaction in dimethyl sulfoxide and subsequent crystallization was found to afford Pb(SePh)₂. Inductively coupled plasma optical emission spectroscopy revealed a 1:2 lead to selenium ratio for all three complexes. Nuclear magnetic resonance spectroscopy confirms that Pb(SePh)₂ is readily solubilized by ethylenediamine, and electrospray ionization mass spectrometry supports the presence of Pb(SePh)₂ moieties in solution. Single-crystal X-ray diffraction analysis of the pyridine adduct, (py)₂Pb(SePh)₂, revealed a seesaw molecular geometry featuring equatorial phenylselenolate ligands. Crystals of Pb(SePh)₂ grown from dimethyl sulfoxide revealed one-dimensional polymeric chains of Pb(SePh)₂. We believe that the lead(II) phenylselenolate complexes form via an oxidative addition reaction.

Scope and mechanism of nitrile dihydroboration mediated by a β-diketiminate manganese hydride catalyst

Nitrile insertion allows for manganese-catalyzed nitrile dihydroboration at 80 °C.

Efficient Cobalt Catalyst for Ambient-Temperature Nitrile Dihydroboration, the Elucidation of a Chelate-Assisted Borylation Mechanism, and a New Synthetic Route to Amides

N,N-Diborylamines have emerged as promising reagents in organic synthesis; however, their efficient preparation and full synthetic utility have yet to be realized. To address both shortcomings, an effective catalyst for nitrile dihydroboration was sought. Heating CoCl₂ in the presence of ^PyEtPDI afforded the six-coordinate Co(II) salt, [(^PyEtPDI)CoCl][Cl]. Upon adding 2 equiv of NaEt₃BH, hydride transfer to one chelate imine functionality was observed, resulting in the formation of (κ⁴-N,N,N,N-^PyEtIP^CHMeN^EtPy)Co. Single-crystal X-ray diffraction and density functional theory calculations revealed that this compound possesses a low-spin Co(II) ground state featuring antiferromagnetic coupling to a singly reduced imino(pyridine) moiety. Importantly, (κ⁴-N,N,N,N-^PyEtIP^CHMeN^EtPy)Co was found to catalyze the dihydroboration of nitriles using HBPin with turnover frequencies of up to 380 h^-1 at ambient temperature. Stoichiometric addition experiments revealed that HBPin adds across the Co-N_amide bond to generate a hydride intermediate that can react with additional HBPin or nitriles. Computational evaluation of the reaction coordinate revealed that the B-H addition and nitrile insertion steps occur on the antiferromagnetically coupled triplet spin manifold. Interestingly, formation of the borylimine intermediate was found to occur following BPin transfer from the borylated chelate arm to regenerate (κ⁴-N,N,N,N-^PyEtIP^CHMeN^EtPy)Co. Borylimine reduction is in turn facile and follows the same ligand-assisted borylation pathway. The independent hydroboration of alkyl and aryl imines was also demonstrated at 25 °C. With a series of N,N-diborylamines in hand, their addition to carboxylic acids allowed for the direct synthesis of amides at 120 °C, without the need for an exogenous coupling reagent.

A β-diketiminate manganese catalyst for alkene hydrosilylation: substrate scope, silicone preparation, and mechanistic insight

A (BDI)Mn catalyst has been found to hydrosilylate olefins and the observed selectivity can be attributed to alkene insertion.

The dimeric β-diketiminate manganese hydride compound, [(^2,6-iPr2PhBDI)Mn(μ-H)]₂, was prepared by treating [(^2,6-iPr2PhBDI)Mn(μ-Cl)]₂ with NaEt₃BH. This compound was characterized by single crystal X-ray diffraction and found to feature high-spin Mn centres that exhibit strong magnetic coupling by EPR spectroscopy. Once characterized, [(^2,6-iPr2PhBDI)Mn(μ-H)]₂ was found to mediate the hydrosilylation of a broad scope of alkenes at elevated temperature. Aliphatic alkenes were found to undergo anti-Markovnikov hydrosilylation, while the hydrosilylation of styrenes using [(^2,6-iPr2PhBDI)Mn(μ-H)]₂ afforded Markovnikov's product. Importantly, this catalyst has also been employed for the cross-linking of industrially-relevant silicones derived from vinyl-terminated poly(dimethylsiloxane) and 1,2,4-trivinylcyclohexane with catalyst loadings as low as 0.05 mol%. To gain a mechanistic understanding of [(^2,6-iPr2PhBDI)Mn(μ-H)]₂-catalyzed olefin hydrosilylation, 4-tert-butylstyrene was added to [(^2,6-iPr2PhBDI)Mn(μ-H)]₂ and conversion to the monomeric Mn alkyl complex, (^2,6-iPr2PhBDI)Mn(CH(CH₃)(4-^tBuPh)), was observed. Isolation of this secondary alkyl intermediate confirms that olefin insertion into the Mn–H bond dictates the observed regioselectivities. The importance of our mechanistic findings as they relate to recent advances in Mn hydrosilylation catalysis is described herein.

Ge0.57Ti0.43O2: a new high-pressure material with rutile-type crystal structure

Single crystals of a GeO2-TiO2 solid solution with the corresponding composition Ge0.57Ti0.43O2 (germanium titanium tetra-oxide) were obtained by devitrification of germania-titania glass at high pressure and temperature. The new compound crystallizes in the rutile structure type (space group P42/mnm), where Ge and Ti share the same position M (site symmetry m.mm), with occupancy values of 0.57 (3) and 0.43 (3), respectively, and one O-atom position (m.2m). The M site is in a sixfold O-atom coordination and, as in the original TiO2 rutile structure, an elongation of the O-M-O bonds along the c-axis direction of the coordination polyhedron and deviation of the angles from 90° lead to a decrease in the coordination symmetry from octa-hedral to tetra-gonal. The Ge and Ti atoms are fully disordered in the structure, which indicates that the rutile structure is surprisingly pliant given the differing sizes of the two cations.

Hydroboration of alkynes and nitriles using an α-diimine cobalt hydride catalyst

Addition of NaEt₃BH to (^Ph2PPrDI)CoCl₂ affords the corresponding monohydride, (^Ph2PPrDI)CoH. X-ray diffraction and DFT calculations indicate that this compound possesses a radical monoanion α-DI chelate and a Co(ii) centre. Notably, (^Ph2PPrDI)CoH catalyzes the hydroboration of alkynes and dihydroboration of nitriles under mild conditions.

Carbonyl and ester C–O bond hydrosilylation using κ4-diimine nickel catalysts

(^Ph2PPrDI)Ni chemoselectively catalyzes α-allyl ester C–O bond hydrosilylation to prepare silyl esters with turnover frequencies of up to 990 h⁻¹.

Isolation of a bis(imino)pyridine molybdenum(i) iodide complex through controlled reduction and interconversion of its reaction products

Analysis of previously reported [((Ph2PPr)PDI)MoI][I] by cyclic voltammetry revealed a reversible wave at -1.20 V vs. Fc(+/0), corresponding to the Mo(ii)/Mo(i) redox couple. Reduction of [((Ph2PPr)PDI)MoI][I] using stoichiometric K/naphthalene resulted in ligand deprotonation rather than reduction to yield a Mo(ii) monoiodide complex featuring a Mo-C bond to the α-position of one imine substituent, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI. Successful isolation of the inner-sphere Mo(i) monoiodide complex, ((Ph2PPr)PDI)MoI, was achieved via reduction of [((Ph2PPr)PDI)MoI][I] with equimolar Na/naphthalene. This complex was found to have a near octahedral coordination geometry by single crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy revealed an unpaired Mo-based electron which is highly delocalized onto the PDI chelate core. Attempts to prepare a Mo(i) monohydride complex upon adding NaEt3BH to ((Ph2PPr)PDI)MoI resulted in disproportionation to yield an equimolar quantity of (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH and newly identified ((Ph2PPr)PDI)MoH2. Independent preparation of ((Ph2PPr)PDI)MoH2 was achieved by adding 2 equiv. NaEt3BH to [((Ph2PPr)PDI)MoI][I] and a minimum hydride resonance T1 of 176 ms suggests that the Mo-bound H atoms are best described as classical hydrides. Interestingly, ((Ph2PPr)PDI)MoH2 can be converted to (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI upon iodomethane addition, while ((Ph2PPr)PDI)MoH2 is prepared from (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI in the presence of excess NaEt3BH. Similarly, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI can be converted to (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH with 1 equiv. of NaEt3BH, while the opposite transformation occurs following iodomethane addition to (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH. Facile interconversion between [((Ph2PPr)PDI)MoI][I], (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH, and ((Ph2PPr)PDI)MoH2 is expected to guide future reactivity studies on this unique set of compounds.

Two-step C–H, C–P bond activation at an α-diimine iron dinitrogen complex

Thermolysis of newly prepared (Ph2PPrDI)Fe(N2) results in chelate C–H and C–P bond activation to yield phosphide-bridged dimers.

Hydrogen production from water using a bis(imino)pyridine molybdenum electrocatalyst

Reduction of [(^Ph2PPrPDI)MoO][PF₆]₂ affords an unusual Mo(ii) oxo compound that mediates the electrocatalytic reduction of water.

Antineoplastic agents. 599. Total synthesis of dolastatin 16

The first 23-step total synthesis of the cyclodepsipeptide dolastatin 16 (1) has been achieved. Synthesis of the dolaphenvaline and dolamethylleuine amino acid units using simplified methods improved the overall efficiency. The formation of the 25-membered macrocycle employing lactonization with 2-methyl-6-nitrobenzoic anhydride completed a key step in the synthesis. Regrettably, the synthetic dolastatin 16 (1), while otherwise identical (by X-ray crystal structure and spectral analyses) with the natural product, did not reproduce the powerful (nanomolar) cancer cell growth inhibition displayed by the natural isolate. Presumably this result can be attributed to conformation(s) of the synthetic dolastatin 16 (1) or to a chemically undetected component isolated with the natural product.

Antineoplastic agents. 595. Structural modifications of betulin and the X-ray crystal structure of an unusual betulin amine dimer

The lupane-type triterpene betulin (1) has been subjected to a series of structural modifications for the purpose of evaluating resultant cancer cell growth inhibitory activity. The reaction sequence 7→11→12 was especially noteworthy in providing a betulin-derived amine dimer. Other unexpected synthetic results included the 11 and 13/14→17 conversions, which yielded an imidazo derivative. X-ray crystal structures of dimer 12 and intermediate 25 are reported. All of the betulin modifications were examined for anticancer activity against the P388 murine and human cell lines. Significant cancer cell growth inhibition was found for 4, 8, 9, 15/16, 19, 20, 24, and 26, which further defines the utility of the betulin scaffold.

Modeling, synthesis and biological evaluation of potential retinoid X receptor-selective agonists: novel halogenated analogues of 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (bexarotene)

The synthesis of halogenated analogues of 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (1), known commonly as bexarotene, and their evaluation for retinoid X receptor (RXR)-specific agonist performance is described. Compound 1 is FDA approved to treat cutaneous T-cell lymphoma (CTCL); however, bexarotene treatment can induce hypothyroidism and elevated triglyceride levels, presumably by disrupting RXR heterodimer pathways for other nuclear receptors. The novel halogenated analogues in this study were modeled and assessed for their ability to bind to RXR and stimulate RXR homodimerization in an RXRE-mediated transcriptional assay as well as an RXR mammalian-2-hybrid assay. In an array of eight novel compounds, four analogues were discovered to promote RXR-mediated transcription with EC(50) values similar to that of 1 and are selective RXR agonists. Our approach also uncovered a periodic trend of increased binding and homodimerization of RXR when substituting a halogen atom for a proton ortho to the carboxylic acid on 1.

2-Fluoro-4-(methoxycarbonyl)benzoic acid

In the crystal of the title compound, C₉H₇FO₄, classical carboxylate inversion dimers are linked by pairs of O—H⋯O hydrogen bonds. The packing is consolidated by C—H⋯F and C—H⋯O interactions. The benzene ring and the methoxycarbonyl group are nearly coplanar, with a dihedral angle of 1.5 (3)° between them, whereas the carboxyl group has a dihedral angle of 20.2 (4)° with respect to the benzene ring.

Methyl (Z)-2-chloro-3-(2-methoxy-carbonyl-phen-yl)prop-2-enoate

In the title compound, C(12)H(11)ClO(4), the propenoate C=C bond is in the Z configuration. The propenoate C=O and C=C groups are essentially coplanar [C=C-C=O torsion angle = 172.4 (3)°] with the O atom synperiplanar to the Cl atom. However, the π systems of the aromatic ring and chloro-propenoate substituent are not coplanar; the corresponding dihedral angle is 51.5 (1)°. The noncoplanarity is likely due to steric inter-actions between the propenoate H atom and the ortho-methoxy-carbonyl group on the aromatic ring. Even in the observed noncoplanar conformation, the ortho C=O to H distance (2.40 Å) is less than the sum of the van der Waals radii of O and H (2.65 Å).

Growth and single-crystal refinement of phase-III potassium nitrate, KNO(3)

Oriented single crystals of the high-temperature phase of KNO(3) (phase III), a ferroelectric compound that may also occur as an atmospheric aerosol particle, were grown at room temperature and pressure by atomizing a solution of KNO(3) in water and allowing droplets to dry on a glass substrate. The crystals are up to 1 mm across and are stable unless mechanically disturbed. There is no evidence of the spontaneous transformation of phase III to the room-temperature stable phase (phase II), even after several months. Single-crystal structure determinations of phase III were obtained at 295 and 123 K. The unit cell regained its room-temperature dimensions after warming from 123 K. The phase-III KNO(3) structure can be viewed as the stacking parallel to the c axis of alternating K atoms and planar NO(3) groups. The NO(3) groups connect the planes of K atoms, where each O is fourfold coordinated to one N and three K. Each K atom has nine O nearest neighbors, with three bonds at 2.813 and six at 2.9092 A. The interatomic K-N-K distance alternates from 5.051 to 3.941 along the c axis. The N-O distances increase from 1.245 (2) A at 295 K to 1.2533 (15) A at 123 K. The nitrate group has a slight non-planarity, with the N atoms 0.011 A above the O plane and directed toward the more distant K of the K-N-K chain.

Modeling, synthesis and biological evaluation of potential retinoid X receptor (RXR) selective agonists: novel analogues of 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (bexarotene)

This report describes the synthesis of analogues of 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (1), commonly known as bexarotene, and their analysis in acting as retinoid X receptor (RXR)-specific agonists. Compound 1 has FDA approval to treat cutaneous T-cell lymphoma (CTCL); however, its use can cause side effects such as hypothyroidism and increased triglyceride concentrations, presumably by disruption of RXR heterodimerization with other nuclear receptors. The novel analogues in the present study have been evaluated for RXR activation in an RXR mammalian-2-hybrid assay as well as an RXRE-mediated transcriptional assay and for their ability to induce apoptosis as well as for their mutagenicity and cytotoxicity. Analysis of 11 novel compounds revealed the discovery of three analogues that best induce RXR-mediated transcriptional activity, stimulate apoptosis, have comparable K(i) and EC(50) values to 1, and are selective RXR agonists. Our experimental approach suggests that rational drug design can develop new rexinoids with improved biological properties.

Antineoplastic agents. 579. Synthesis and cancer cell growth evaluation of E-stilstatin 3: a resveratrol structural modification

As an extension of our earlier structure/activity investigation of resveratrol (1a) cancer cell growth inhibitory activity compared to the structurally related stilbene combretastatin series (e.g., 2a), an efficient synthesis of E-stilstatin 3 (3a) and its phosphate prodrug 3b was completed. The trans-stilbene 3a was obtained using a convergent synthesis employing a Wittig reaction with phosphonium bromide 9 as the key reaction step. Deprotection of the Z-silyl ether 13 gave E-stilstatin 3 (3a) as the exclusive product. The structure and stereochemistry of 3a was confirmed by X-ray crystal structure determination.

Zn(5)Sb(4)In(2-delta) - a ternary derivative of thermoelectric zinc antimonides

Zn(5)Sb(4)In(2-delta) (delta = 0.15(3)) was synthesized in the form of millimeter-sized crystals from reaction mixtures containing excess zinc. The ternary intermetallic compound is temperature polymorphic, and at room temperature it crystallizes with a new structure type in the orthorhombic space group Pbcn, where a = 7.1619(2), b = 17.1562(4), c = 8.6887(4) A, V = 1067.6(1) A(3), and Z = 4. The structure features 3(2)434 nets of Sb atoms that are stacked in antiposition to yield layers of square antiprisms sharing edges plus intervening tetracapped tetrahedra (tetreadersterns). The majority of Zn atoms occupy peripheral tetrahedra of such tetraedersterns, and attain at the same time the peculiar five-coordination by one like atom and four Sb atoms typical for the structures of binary zinc antimonides. The In and remaining Zn atoms are distributed in the tetragonal channels formed by the square antiprisms and display some disorder. At temperatures below 200 K Zn(5)Sb(4)In(2-delta) undergoes a phase transition into a more ordered structure with monoclinic symmetry (P2(1)/c) without any change of the unit cell. The thermoelectric properties of Zn(5)Sb(4)In(2-delta) were measured between 10 and 350 K. Exceptionally low thermal conductivity values (1 W/mK range) were obtained in the whole temperature range. Resistivity and thermopower values are characteristic of a heavily doped or degenerate semiconductor (2.5 mOmega cm and 160 muV/K, respectively, at room temperature) and show a discontinuity around 220 K. The thermoelectric figure of merit of Zn(5)Sb(4)In(2-delta) is higher than that of Zn(4)Sb(3) in the investigated temperature range.

Redetermination of tetrakis(trimethylstannyl)germane

Redetermination of the structure of the title compound, [Ge(SnMe₃)₄] or [GeSn₄(CH₃)₁₂], previously refined from powder diffraction data only [Dinnebier, Bernatowicz, Helluy, Sebald, Wunschel, Fitch & van Smaalen et al. (2002 ▶). Acta Cryst. B58, 52–61], confirms that four bulky trimethyl­stannyl ligands surround the central Ge atom (site symmetry 1) in a tetra­hedral coordination.

Di-μ-chlorido-bis[dichlorido(N,N-diethylacetamidinato)(N,N-diethylacetamidine)titanium(IV)] acetonitrile disolvate

In the centrosymmetric title compound [Ti₂Cl₆(C₆H₁₃N₂)₂(C₆H₁₄N₂)₂]·2C₂H₃N, an inversion center relates the two Ti atoms which display a distorted octa­hedral coordination geometry. There are two uncoordinated acetonitrile solvent mol­ecules per mol­ecule of title compound in the crystal structure.